1. Field of the Invention
The present invention relates to image processing techniques and, in particular, to methods and apparatuses for detecting luma/chroma edges of images.
2. Description of the Prior Art
The composite video baseband signal (CVBS) is a currently widely used analog video signal, and it is composed of luma signals and chroma signals of images.
Please refer to FIG. 1(A), which illustrates an example of the CVBS waveform. Region 10 includes plural synchronizing pulses for indicating the start of each horizontal scan line. Region 12 includes sinusoidal signals (generally called the color burst) that provide reference voltages for chroma signals. Region 14 is the portion that includes image data of the CVBS. The voltage of the stair-like luma signal in region 14 corresponds to the brightness of images. For instance, VMAX in FIG. 1(A) may correspond to the brightness of a gray level equal to 255, and VMIN may correspond to the brightness of a gray level equal to 0.
In the CVBS specification, the chroma signal is represented by sinusoidal waves (not shown in the figure) added on the luma signal in region 14. The chroma signal is generated by modulating a carrier signal that has the same frequency as the color burst. By monitoring the phase difference between the chroma signal in region 14 and the color burst in region 12, the receiving end can determine the hue of the chroma signal. As shown in FIG. 1(B), in the frequency domain, the luma signal of the CVBS is distributed in the region of lower frequency, and the chroma signal is distributed in the region of higher frequency.
After receiving the CVBS, most image processing system first separate the luma and chroma signals therein by a luma/chroma separating procedure (also called Y/C separating procedure). In the simplest luma/chroma separating procedure, luma signals are generated by filtering the CVBS with a low-pass filter. On the contrary, chroma signals are generated by filtering the CVBS with a high-pass filter.
However, when a rapid change occurs in the colors and/or brightness of an image, a rapid change in the corresponding luma/chroma signals will also introduce high-frequency components. In other words, the output of the high-pass filter might not be purely chroma signals. More specifically, some non-chroma components may be misjudged as chroma components. This condition generally happens at intersections of different colors and/or brightness in an image.
To prevent the aforementioned misjudging problem, luma/chroma edge regions must be avoided when the CVBS is filtered in the luma/chroma separating procedure. Therefore, before the luma/chroma separating procedure, the receiving end usually must first perform an edge detecting procedure in advance, so as to avoid or neglect edge regions.
Besides the luma/chroma separating procedure, some image processing systems also perform a sharpening procedure on the CVBS. The sharpening procedure needs information about edge positions as well.
Please refer to FIG. 2, which illustrates the block diagram of a conventional CVBS receiving system. The receiving system 20 includes a sampling module 21, a buffer 22, an edge detecting module 23, a luma/chroma separating module 24, and a sharpening module 25.
The sampling module 21 first samples the CVBS transmitted to the receiving system 20 and temporarily stores the sampling results into the buffer 22. The data stored in the buffer 22 may correspond to a complete image or only a block in an image. The edge detecting module 23 first judges whether any edge exists in the image/block stored in the buffer 22 and then provides its judgment to the luma/chroma separating module 24 and the sharpening module 25.
Please refer to FIG. 3(A), which illustrates an example of a CVBS and several sample points (32A˜32E). In this example, the frequency of the sampling signal is four times of the frequency of the color burst. The label T in this figure represents the period of the color burst. Theoretically, if there is no rapid change in colors and/or brightness of the image, the voltages of the two sample points with a time difference of T should be approximately the same. In other words, the voltages of the sample points 32A and 32E should be about equivalent.
Please refer to FIG. 3(B), which illustrates an example of another CVBS and several sample points (34A˜34E). In this example, the brightness of the CVBS changes between the sample points 34C and 34D. Thus, the voltage difference of the sample points 34A and 34E is larger. Through comparing the voltages of the sample points 34A and 34E, the edge detecting module 23 can identify that an edge of colors and/or brightness exists between the sample points 34A and 34E.
In prior arts, limited by the characteristic of sinusoidal signals, the sample time of two sample points to be compared must be separated by time T or a multiple of T. In other words, the highest “resolution” of conventional edge detecting procedures is larger than or equal to the period of the chroma signal. Accordingly, under the condition shown in FIG. 3(B), the receiving end can only identify that at least one edge exists between the sample points 34A and 34E but cannot judge the exact position of the edge.
When the colors and/or brightness of an image rapidly changes, and the changing period is smaller than that of the color burst, conventional edge detecting procedures may even be unable to detect the existence of the edges. If an edge detecting procedure is unable to find out edges correctly, the image quality provided by the receiving end may be significantly decreased.